Conventional methods of detecting the position or speed of an moving object can be roughly classified into methods using magnetic encoders and methods using optical encoders. The optical encoder is mainly constituted by a light-emitting portion, light-receiving portion, and scale. The scale is generally manufactured by precision press-cutting or etching by using a thin SUS (stainless steel) plate. This scale corresponds to the one disclosed in Japanese Utility Model Laid-Open No. 63-33409.
According to the scale manufactured by the above conventional technique, a hub portion required to fix the scale to a motor shaft is formed as a discrete component. In order to guarantee coaxial mounting between the scale slit portion and hub portion, the scale and hub must be adjusted when they are attached, requiring much time and labor.
As a conventional scale for an optical rotary encoder, a metal scale obtained by forming a slit portion in a metal plate having a thickness of about 0.05 mm to 0.2 mm by etching or the like, or a film scale obtained by forming a light-transmitting portion and light-absorbing portion on a photoengraving film having a thickness of about 0.2 mm by using a means such as photographic development is known. As a high-precision scale used in a field demanding high precision, a glass scale obtained by depositing a thin metal film on a glass substrate and forming a slit portion by photoetching or the like is known.
Each scale described above indirectly coupled to the coupling portion of a rotating shaft through a hub member. It is difficult to fix the scale to the shaft while maintaining high eccentricity precision between the center of the slit portion formed in the scale and the rotating shaft. This requires high-precision adjustment. Likewise, owing to the above arrangement, it is difficult to keep high squareness precision between the scale and the rotating shaft in the presence of wobbling of the scale surface upon rotation. Furthermore, since a metal scale or film scale has a thickness of 0.2 mm or less, it exhibits poor flatness. As a consequence, the scale suffers from large wobbling (flapping) of the surface. This becomes a factor that causes a deterioration in angle detection precision. It is relatively easy for a glass scale, from which relatively high flatness can be obtained, to improve squareness precision. However, this scale is susceptible to shock, and expensive.
Attempts to avoid such factors that cause deterioration in precision have interfered with reductions in the size and cost of encoder units. For example, with regard to the size of an encoder in the direction of thickness (shaft thrust direction), to minimize wobbling of the surface, the scale mount hub needs to be long. This increases the overall size of the encoder unit in the thrust direction, interfering with a reduction in the profile of the encoder. In addition, in order to couple/assemble the above components while high coaxiality and squareness are maintained, a high-precision adjustment process is required, resulting in an increase in cost.